This application is based upon and claims the benefit of Japanese Patent Applications No. 2001-357453 filed on Nov. 22, 2001 and No. 2001-357454 filed on Nov. 22, 2001, the contents of which are incorporated herein by reference.
The present invention relates to a band gap reference voltage circuit that outputs a constant output voltage.
Conventionally, referring to FIG. 12A, a band gap reference voltage circuit J1 is used as a circuit for generating a reference voltage within an IC. For example, the voltage circuit J1 may be formed at a predetermined portion on the IC chip J2. The IC chip 12 is, as shown in FIG. 12B, mounted on and electrically connected to a lead frame 13 via wire 14, and further is encapsulated within molded resin 15 having fillers.
When a semiconductor device encapsulated within molded resin J5 and including the IC chip J2 with the band gap reference voltage circuit J1 is manufactured, an output voltage of the reference voltage circuit J1 decreases due to a change in circuit characteristics. Further, the amount by which the output voltage decreases has been shown to be non-uniform for different samples.
A graph of experimental results is shown in FIG. 13. The output voltage does not change immediately after the IC chip J2 is encapsulated but changes with respect to stress applied to the IC chip J2 as the resin dries.
In this case, the band gap reference voltage circuit J1 cannot be used as the circuit for generating a reference voltage in a semiconductor device in which high accuracy is required.
Generally, electrical properties of the IC chip change when stress is applied to the IC chip. When stress is applied to the band gap reference voltage circuit J1, voltages VBE of transistors (or voltages VF of diodes) thereof decrease, thereby decreasing the output voltage.
Therefore, it is undesirable that any stress is applied to the band gap reference voltage circuit J1. Through experimentation, it has been confirmed that the output voltage of the band gap reference voltage circuit J1 returns a value before being encapsulated if the resin is removed from the IC chip J2, and the output voltage decrease and non-uniformity is generated regardless of a position of the band gap reference voltage circuit J1. These results suggest that stress is applied over the entire area of the IC chip J2 by the resin.
Since fillers in the resin apparently cause the output voltage decrease, a semiconductor device encapsulated within resin having no fillers in which the IC chip J2 including the band gap reference voltage circuit J1 is accommodated is manufactured. As a result, the output voltage of the band gap reference voltage circuit J1 encapsulated within only resin is smaller than that of the band gap reference voltage circuit J1 encapsulated within resin having fillers. The result suggests that the fillers are related to the output voltage decrease and non-uniformity.
For example, the output voltage decrease may be caused by compression stress applied to a surface of the IC chip J2 in a direction perpendicular thereto because many kinds of stresses are concentrically applied due to the fillers touching the surface of the IC chip J2. In addition, sizes of the fillers are not identical, densities of the fillers in the resin are different with respect to position thereof, and a number of the fillers touching the band gap reference voltage circuit J1 depends on a total area of the band gap reference voltage circuit J1. Therefore, the compression stress may vary based on these factors, thereby increasing non-uniformity of the output voltage.
Therefore, the output voltage decrease and non-uniformity can be decreased by removing the fillers from the resin. However, since a thermal expansion coefficient of the resin is higher than that of the IC chip J2 or lead frame J3 if the fillers are removed, the band gap reference voltage circuit J1 cannot be used for technical applications that require a wide operation temperature range.
In the band gap reference voltage circuit J1, it is generally preferable that a constant reference voltage is output even if the atmospheric temperature changes. However, since transistors or the like included in the band gap reference voltage circuit J1 have temperature characteristics, the band gap reference voltage circuit J1 has a quadric factor with respect to temperature. FIG. 14 shows a relationship between temperature and an output voltage of a related band gap reference voltage circuit. The relationship is illustrated by parabolically-shaped curve of FIG. 14. When the band gap reference voltage circuit J1 is applied to, for example, a circuit for forming the reference voltage of a high accuracy power supply, an accuracy of the reference voltage is insufficient due to the quadric factor with respect to temperature, and therefore more accurate reference voltage is required.
It is therefore an object of the present invention to provide a band gap reference voltage circuit that is capable of obviating the above problem.
It is another object of the present invention to provide a band gap reference voltage circuit that is capable of minimizing output voltage decrease and non-uniformity even if fillers are included in resin.
It is further object of the present invention to provide a band gap reference voltage circuit that is capable of suppressing output voltage variation due to temperature change.
According to a band gap reference voltage circuit of the present invention, a first band gap reference voltage formation portion outputs a first reference voltage. A second band gap reference voltage formation portion outputs a second reference voltage. The band gap reference voltage circuit then outputs a higher one of the first and second reference voltages of the first and second band gap reference voltage formation portions to output a constant reference voltage.
Accordingly, even if one of the first and second reference voltages decreases due to application of stress to the first and second band gap reference voltage formation portions, the higher of the two voltages is output as an output voltage of the band gap reference voltage circuit. As a result, the output voltage of the band gap reference voltage circuit is almost constant, and therefore a band gap reference voltage circuit with low output voltage decrease and non-uniformity can be obtained when it is, for example, encapsulated within resin having fillers.
According to a band gap reference voltage circuit of the present invention, a first level shift circuit shifts a temperature on which the second reference voltage of the second band gap reference voltage formation portion has a maximum value from a temperature on which the first reference voltage of the first band gap reference voltage formation portion has a maximum value.
Accordingly, for example, the band gap reference voltage circuit outputs a first reference voltage of the first band gap reference voltage formation portion as the output voltage in a low temperature range and a second reference voltage of the second band gap reference voltage formation portion as the output voltage in a high temperature range. As a result, the output voltage is formed by a composite voltage of the first and second reference voltages of the first and second band gap reference voltage formation portions in low and high temperature ranges so that the variation of the output voltage is suppressed. Therefore, the output voltage is approximately constant over wide temperature range, and therefore a band gap reference voltage circuit with low variation in output voltage can be obtained even if atmospheric temperature varies.